Pace counter isolation for cardiac resynchronization pacing

ABSTRACT

A system and method recording sensing and pacing events in a cardiac rhythm management device. The method may be particularly useful in assessment of pacing parameters for ventricular resynchronization therapy.

FIELD OF THE INVENTION

[0001] This invention pertains to methods and apparatus for cardiacrhythm management. In particular, the invention relates to methods andapparatus for providing cardiac resynchronization therapy.

BACKGROUND

[0002] Cardiac rhythm management devices are implantable devices thatprovide electrical stimulation to selected chambers of the heart inorder to treat disorders of cardiac rhythm and include pacemakers andimplantable cardioverter/defibrillators. A pacemaker is a cardiac rhythmmanagement device that paces the heart with timed pacing pulses. Themost common condition for which pacemakers are used is in the treatmentof bradycardia, where the ventricular rate is too slow.Atrio-ventricular conduction defects (i.e., AV block) that are permanentor intermittent and sick sinus syndrome represent the most common causesof bradycardia for which permanent pacing may be indicated. Iffunctioning properly, the pacemaker makes up for the heart's inabilityto pace itself at an appropriate rhythm in order to meet metabolicdemand by enforcing a minimum heart rate. Pacing therapy may also beapplied in order to treat cardiac rhythms that are too fast, termedanti-tachycardia pacing. (As the term is used herein, a pacemaker is anycardiac rhythm management device with a pacing functionality, regardlessof any other functions it may perform such as the delivery cardioversionor defibrillation shocks to terminate atrial or ventricularfibrillation.)

[0003] Also included within the concept of cardiac rhythm is the degreeto which the heart chambers contract in a coordinated manner during acardiac cycle to result in the efficient pumping of blood. The heart hasspecialized conduction pathways in both the atria and the ventriclesthat enable the rapid conduction of excitation (i.e., depolarization)throughout the myocardium. These pathways conduct excitatory impulsesfrom the sinoatrial node to the atrial myocardium, to theatrio-ventricular node, and thence to the ventricular myocardium toresult in a coordinated contraction of both atria and both ventricles.This both synchronizes the contractions of the muscle fibers of eachchamber and synchronizes the contraction of each atrium or ventriclewith the contralateral atrium or ventricle. Without the synchronizationafforded by the normally functioning specialized conduction pathways,the heart's pumping efficiency is greatly diminished. Patients whoexhibit pathology of these conduction pathways, such as bundle branchblocks, can thus suffer compromised cardiac output.

[0004] Patients with conventional pacemakers can also have compromisedcardiac output because artificial pacing with an electrode fixed into anarea of the myocardium does not take advantage of the above-describedspecialized conduction system. The spread of excitation from a singlepacing site must proceed only via the much slower conducting musclefibers of either the atria or the ventricles, resulting in the part ofthe myocardium stimulated by the pacing electrode contracting wellbefore parts of the chamber located more distally to the electrode,including the myocardium of the chamber contralateral to the pacingsite. Although the pumping efficiency of the heart is somewhat reducedfrom the optimum, most patients can still maintain more than adequatecardiac output with artificial pacing.

[0005] Heart failure is a clinical syndrome in which an abnormality ofcardiac function causes cardiac output to fall below a level adequate tomeet the metabolic demand of peripheral tissues and is usually referredto as congestive heart failure (CHF) due to the accompanying venous andpulmonary congestion. CHF can be due to a variety of etiologies withischemic heart disease being the most common. Some CHF patients sufferfrom some degree of AV block or are chronotropically deficient such thattheir cardiac output can be improved with conventional bradycardiapacing. Such pacing, however, may result in some degree ofuncoordination in atrial and/or ventricular contractions due to the wayin which pacing excitation is spread throughout the myocardium asdescribed above. The resulting diminishment in cardiac output may besignificant in a CHF patient whose cardiac output is alreadycompromised. Intraventricular and/or interventricular conduction defects(e.g., bundle branch blocks) are also commonly found in CHF patients. Inorder to treat these problems, cardiac rhythm management devices havebeen developed which provide electrical pacing stimulation to one ormore heart chambers in an attempt to improve the coordination of atrialand/or ventricular contractions, termed cardiac resynchronizationtherapy.

[0006] In order for cardiac resynchronization therapy to be effective,resynchronization pacing pulses should be delivered as often aspossible. If the pacemaker is operating in a mode where pacing isinhibited by intrinsic cardiac activity, this means that a pace must bedelivered before such intrinsic activation takes place. Pacemakers havevarious programmable pacing parameters that affect the extent to whichpaces are delivered and not inhibited by intrinsic beats. In order tooptimally adjust these parameters, an informative record of sensing andpacing events over a period of time is needed. It is toward this generalproblem that the present invention is directed.

SUMMARY OF THE INVENTION

[0007] The present invention is a system and method for recordingsensing and pacing events in a cardiac pacemaker that provides usefulinformation for adjusting pacing parameters in order to optimallydeliver cardiac resynchronization therapy. In accordance with theinvention, paces delivered to a heart chamber occurring as a result ofthe expiration of different escape intervals or trigger events areseparately counted. Each event that may cause a pace is assigned anisolated counter to count the number of paces that occur by reason ofthat event. For example, in the case of ventricular pacing, separatecounters may be provided for expiration of the lower rate limitventricular escape interval, the atrio-ventricular interval expirationin atrial-tracking ventricular pacing modes, and a ventricular sense inthe case of a ventricular-triggered mode. The count of the paces due toeach event may be expressed as a percentage of total cardiac cycles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a system diagram of a pacemaker configured forbiventricular pacing and sensing.

[0009]FIGS. 2 through 5 illustrates examples of data produced byseparate and nonseparate pace counters.

DESCRIPTION OF THE INVENTION

[0010] In order to properly deliver ventricular resynchronizationtherapy, it is important to pace the ventricles to as great an extent aspossible. If the pacemaker is operating in a synchronous mode wherepaces are inhibited by intrinsic activity, this can be brought about byoptimal adjustment of pacing parameters such as the length of escapeintervals. A clinician may properly set the parameters initially, butchanges in the patient's condition over time may render those parametervalues inappropriate for optimally delivering resynchronization therapy.Pacemakers typically collect diagnostic data over time which can bedownloaded by an external programmer using a data link. This dataincludes counts of senses received from sensing channels and counts ofpaces delivered. In order to optimally configure the pacemaker fordelivering resynchronization therapy, however, more information thanthat is needed. Specifically, information is needed that enables aclinician to determine the reasons why paces are or are not beingdelivered. The present invention provides this by separately countingpacing events using isolated pace counters.

[0011] 1. Hardware Platform

[0012] Pacemakers are typically implanted subcutaneously and have leadsthreaded intravenously into the heart to connect the device toelectrodes used for sensing and pacing. A programmable electroniccontroller causes the pacing pulses to be output in response to lapsedtime intervals and sensed electrical activity (i.e., intrinsic heartbeats not as a result of a pacing pulse). Pacemakers sense intrinsiccardiac electrical activity by means of internal electrodes disposednear the chamber to be sensed. A depolarization wave associated with anintrinsic contraction of the atria or ventricles that is detected by thepacemaker is referred to as an atrial sense or ventricular sense,respectively. In order to cause such a contraction in the absence of anintrinsic beat, a pacing pulse (either an atrial pace or a ventricularpace) with energy above a certain pacing threshold is delivered to thechamber.

[0013]FIG. 1 shows a system diagram of a microprocessor-based pacemakerphysically configured with sensing and pacing channels for both atriaand both ventricles. The controller 10 of the pacemaker is amicroprocessor which communicates with a memory 12 via a bidirectionaldata bus. The memory 12 typically comprises a ROM (read-only memory) forprogram storage and a RAM (random-access memory) for data storage. Thepacemaker has atrial sensing and pacing channels comprising electrode 34a-b, leads 33 a -b, sensing amplifiers 31 a-b, pulse generators 32 a-b,and atrial channel interfaces 30 a-b which communicate bidirectionallywith microprocessor 10. The device also has ventricular sensing andpacing channels for both ventricles comprising electrodes 24 a-b, leads23 a-b, sensing amplifiers 21 a-b, pulse generators 22 a-b, andventricular channel interfaces 20 a-b. In the figure, “a” designates oneventricular or atrial channel and “b” designates the channel for thecontralateral chamber. In this embodiment, a single electrode is usedfor sensing and pacing in each channel, known as a unipolar lead. Otherembodiments may employ bipolar leads which include two electrodes foroutputting a pacing pulse and/or sensing intrinsic activity. The channelinterfaces 20 a-b and 30 a-b include analog-to-digital converters fordigitizing sensing signal inputs from the sensing amplifiers andregisters which can be written to by the microprocessor in order tooutput pacing pulses, change the pacing pulse amplitude, and adjust thegain and threshold values for the sensing amplifiers. An exertion levelsensor 330 (e.g., an accelerometer or a minute ventilation sensor)enables the controller to adapt the pacing rate in accordance withchanges in the patient's physical activity. A telemetry interface 40 isalso provided for communicating with an external programmer 500 whichhas an associated display 510. A pacemaker incorporating the presentinvention may possess all of the components in FIG. 1 and beprogrammable so as to operate in a number of different modes, or it mayhave only those components necessary to operate in a particular mode.

[0014] The controller 10 controls the overall operation of the device inaccordance with programmed instructions stored in memory. The controller10 controls the delivery of paces via the pacing channels, interpretssense signals from the sensing channels, implements timers for definingescape intervals and sensory refractory periods, and performs the pacecounting functions as described below. It should be appreciated,however, that these functions could also be performed by custom logiccircuitry either in addition to or instead of a programmedmicroprocessor.

[0015] 2. Bradycardia Pacing Modes

[0016] Bradycardia pacing modes refer to pacing algorithms used to pacethe atria and/or ventricles when the intrinsic atrial and/or ventricularrate is inadequate due to, for example, AV conduction blocks or sinusnode dysfunction. Such modes may either be single-chamber pacing, whereeither an atrium or a ventricle is paced, or dual-chamber pacing inwhich both an atrium and a ventricle are paced. The modes are generallydesignated by a letter code of three positions where each letter in thecode refers to a specific function of the pacemaker. The first letterrefers to which heart chambers are paced and which may be an A (foratrium), a V (for ventricle), D (for both chambers), or O (for none).The second letter refers to which chambers are sensed by the pacemaker'ssensing channels and uses the same letter designations as used forpacing. The third letter refers to the pacemaker's response to a sensedP wave from the atrium or an R wave from the ventricle and may be an I(for inhibited), T (for triggered), D (for dual in which both triggeringand inhibition are used), and O (for no response). Modern pacemakers aretypically programmable so that they can operate in any mode which thephysical configuration of the device will allow. Additional sensing ofphysiological data allows some pacemakers to change the rate at whichthey pace the heart in accordance with some parameter correlated tometabolic demand. Such pacemakers are called rate-adaptive pacemakersand are designated by a fourth letter added to the three-letter code, R.

[0017] Pacemakers can enforce a minimum heart rate either asynchronouslyor synchronously. In asynchronous pacing, the heart is paced at a fixedrate irrespective of intrinsic cardiac activity. There is thus a riskwith asynchronous pacing that a pacing pulse will be deliveredcoincident with an intrinsic beat and during the heart's vulnerableperiod which may cause fibrillation. Most pacemakers for treatingbradycardia today are therefore programmed to operate synchronously in aso-called demand mode where sensed cardiac events occurring within adefined interval either trigger or inhibit a pacing pulse. Inhibiteddemand pacing modes utilize escape intervals to control pacing inaccordance with sensed intrinsic activity. In an inhibited demand mode,a pacing pulse is delivered to a heart chamber during a cardiac cycleonly after expiration of a defined escape interval during which nointrinsic beat by the chamber is detected. If an intrinsic beat occursduring this interval, the heart is thus allowed to “escape” from pacingby the pacemaker. Such an escape interval can be defined for each pacedchamber. For example, a ventricular escape interval can be definedbetween ventricular events so as to be restarted with each ventricularsense or pace. The inverse of this escape interval is the minimum rateat which the pacemaker will allow the ventricles to beat, sometimesreferred to as the lower rate limit (LRL).

[0018] In atrial tracking pacemakers (i.e., VDD or DDD mode), anotherventricular escape interval is defined between atrial and ventricularevents, referred to as the atrio-ventricular interval (AVI). Theatrio-ventricular interval is triggered by an atrial sense or pace andstopped by a ventricular sense or pace. A ventricular pace is deliveredupon expiration of the atrio-ventricular interval if no ventricularsense occurs before. Atrial-tracking ventricular pacing attempts tomaintain the atrio-ventricular synchrony occurring with physiologicalbeats whereby atrial contractions augment diastolic filling of theventricles. If a patient has a physiologically normal atrial rhythm,atrial-tracking pacing also allows the ventricular pacing rate to beresponsive to the metabolic needs of the body.

[0019] A pacemaker can also be configured to pace the atria on aninhibited demand basis. An atrial escape interval is then defined as themaximum time interval in which an atrial sense must be detected after aventricular sense or pace before an atrial pace will be delivered. Whenatrial inhibited demand pacing is combined with atrial-triggeredventricular demand pacing (i.e., DDD mode), the lower rate limitinterval is then the sum of the atrial escape interval and theatrio-ventricular interval.

[0020] Another type of synchronous pacing is atrial-triggered orventricular-triggered pacing. In this mode, an atrium or ventricle ispaced immediately after an intrinsic beat is detected in the respectivechamber. Triggered pacing of a heart chamber is normally combined withinhibited demand pacing so that a pace is also delivered upon expirationof an escape interval in which no intrinsic beat occurs. Such triggeredpacing may be employed as a safer alternative to asynchronous pacingwhen, due to far-field sensing of electromagnetic interference fromexternal sources or skeletal muscle, false inhibition of pacing pulsesis a problem. If a sense in the chamber's sensing channel is an actualdepolarization and not a far-field sense, the triggered pace isdelivered during the chamber's physiological refractory period and is ofno consequence.

[0021] Finally, rate-adaptive algorithms may be used in conjunction withbradycardia pacing modes. Rate-adaptive pacemakers modulate theventricular and/or atrial escape intervals based upon measurementscorresponding to physical activity. Such pacemakers are applicable tosituations in which atrial tracking modes cannot be used. In arate-adaptive pacemaker, for example, the LRL is adjusted in accordancewith exertion level measurements such as from an accelerometer or minuteventilation sensor in order for the heart rate to more nearly matchmetabolic demand. The adjusted LRL is then termed the sensor-indicatedrate.

[0022] 3. Cardiac Resynchronization Therapy

[0023] Cardiac resynchronization therapy is pacing stimulation appliedto one or more heart chambers in a manner that restores or maintainssynchronized bilateral contractions of the atria and/or ventricles andthereby improves pumping efficiency. Certain patients with conductionabnormalities may experience improved cardiac synchronization withconventional single-chamber or dual-chamber pacing as described above.For example, a patient with left bundle branch block may have a morecoordinated contraction of the ventricles with a pace than as a resultof an intrinsic contraction. In that sense, conventional bradycardiapacing of an atrium and/or a ventricle may be considered asresynchronization therapy. Resynchronization pacing, however, may alsoinvolve pacing both ventricles and/or both atria in accordance with asynchronized pacing mode as described below. A single chamber may alsobe resynchronized to compensate for intra-atrial or intra-ventricularconduction delays by delivering paces to multiple sites of the chamber.

[0024] It is advantageous to deliver resynchronization therapy inconjunction with one or more synchronous bradycardia pacing modes, suchas are described above. One atrial and/or one ventricular pacing sitesare designated as rate sites, and paces are delivered to the rate sitesbased upon pacing and sensed intrinsic activity at the site inaccordance with the bradycardia pacing mode. In a single-chamberbradycardia pacing mode, for example, one of the paired atria or one ofthe ventricles is designated as the rate chamber. In a dual-chamberbradycardia pacing mode, either the right or left atrium is selected asthe atrial rate chamber and either the right or left ventricle isselected as the ventricular rate chamber. The heart rate and the escapeintervals for the pacing mode are defined by intervals between sensedand paced events in the rate chambers only. Resynchronization therapymay then be implemented by adding synchronized pacing to the bradycardiapacing mode where paces are delivered to one or more synchronized pacingsites in a defined time relation to one or more selected sensing andpacing events that either reset escape intervals or trigger paces in thebradycardia pacing mode. Multiple synchronized sites may be pacedthrough multiple synchronized sensing/pacing channels, and the multiplesynchronized sites may be in the same or different chambers as the ratesite.

[0025] In bilateral synchronized pacing, which may be either biatrial orbiventricular synchronized pacing, the heart chamber contralateral tothe rate chamber is designated as a synchronized chamber. For example,the right ventricle may be designated as the rate ventricle and the leftventricle designated as the synchronized ventricle, and the paired atriamay be similarly designated. Each synchronized chamber is then paced ina timed relation to a pace or sense occurring in the contralateral ratechamber.

[0026] One synchronized pacing mode may be termed offset synchronizedpacing. In this mode, the synchronized chamber is paced with a positive,negative, or zero timing offset as measured from a pace delivered to itspaired rate chamber, referred to as the synchronized chamber offsetinterval. The offset interval may be zero in order to pace both chamberssimultaneously, positive in order to pace the synchronized chamber afterthe rate chamber, or negative to pace the synchronized chamber beforethe rate chamber. One example of such pacing is biventricular offsetsynchronized pacing where both ventricles are paced with a specifiedoffset interval. The rate ventricle is paced in accordance with asynchronous bradycardia mode which may include atrial tracking, and theventricular escape interval is reset with either a pace or a sense inthe rate ventricle. (Resetting in this context refers to restarting theinterval in the case of an LRL ventricular escape interval and tostopping the interval in the case of an AVI.) Thus, a pair ofventricular paces are delivered after expiration of the AVI escapeinterval or expiration of the LRL escape interval, with ventricularpacing inhibited by a sense in the rate ventricle that restarts the LRLescape interval and stops the AVI escape interval. In this mode, thepumping efficiency of the heart will be increased in some patients bysimultaneous pacing of the ventricles with an offset of zero. However,it may be desirable in certain patients to pace one ventricle before theother in order to compensate for different conduction velocities in thetwo ventricles, and this may be accomplished by specifying a particularpositive or negative ventricular offset interval.

[0027] Another synchronized mode is triggered synchronized pacing. Inone type of triggered synchronized pacing, the synchronized chamber ispaced after a specified trigger interval following a sense in the ratechamber, while in another type the rate chamber is paced after aspecified trigger interval following a sense in the synchronizedchamber. The two types may also be employed simultaneously. For example,with a trigger interval of zero, pacing of one chamber is triggered tooccur in the shortest time possible after a sense in the other chamberin order to produce a coordinated contraction. (The shortest possibletime for the triggered pace is limited by a sense-to-pace latency perioddictated by the hardware.) This mode of pacing may be desirable when theintra-chamber conduction time is long enough that the pacemaker is ableto reliably insert a pace before depolarization from one chamber reachesthe other. Triggered synchronized pacing can also be combined withoffset synchronized pacing such that both chambers are paced with thespecified offset interval if no intrinsic activity is sensed in the ratechamber and a pace to the rate chamber is not otherwise delivered as aresult of a triggering event. A specific example of this mode isventricular triggered synchronized pacing where the rate andsynchronized chambers are the right and left ventricles, respectively,and a sense in the right ventricle triggers a pace to the left ventricleand/or a sense in the left ventricle triggers a pace to the rightventricle.

[0028] As with other synchronized pacing modes, the rate chamber in atriggered synchronized pacing mode can be paced with one or moresynchronous bradycardia pacing modes. If the rate chamber is controlledby a triggered bradycardia mode, a sense in the rate chamber sensingchannel, in addition to triggering a pace to the synchronized chamber,also triggers an immediate rate chamber pace and resets any rate chamberescape interval. The advantage of this modal combination is that thesensed event in the rate chamber sensing channel might actually be afar-field sense from the synchronized chamber, in which case the ratechamber pace should not be inhibited. In a specific example, the rightand left ventricles are the rate and synchronized chambers,respectively, and a sense in the right ventricle triggers a pace to theleft ventricle. If right ventricular triggered pacing is also employedas a bradycardia mode, both ventricles are paced after a rightventricular sense has been received to allow for the possibility thatthe right ventricular sense was actually a far-field sense of leftventricular depolarization in the right ventricular channel. If theright ventricular sense were actually from the right ventricle, theright ventricular pace would occur during the right ventricle'sphysiological refractory period and cause no harm.

[0029] As mentioned above, certain patients may experience some cardiacresynchronization from the pacing of only one ventricle and/or oneatrium with a conventional bradycardia pacing mode. It may be desirable,however, to pace a single atrium or ventricle in accordance with apacing mode based upon senses from the contralateral chamber. This mode,termed synchronized chamber-only pacing, involves pacing only thesynchronized chamber based upon senses from the rate chamber. One way toimplement synchronized chamber-only pacing is to pseudo-pace the ratechamber whenever the synchronized chamber is paced before the ratechamber is paced, such that the pseudo-pace inhibits a rate chamber paceand resets any rate chamber escape intervals. Such pseudo-pacing can becombined with the offset synchronized pacing mode using a negativeoffset to pace the synchronized chamber before the rate chamber and thuspseudo-pace the rate chamber, which inhibits the real scheduled ratechamber pace and resets the rate chamber pacing escape intervals. Oneadvantage of this combination is that sensed events in the rate chamberwill inhibit the synchronized chamber-only pacing, which may benefitsome patients by preventing pacing that competes with intrinsicactivation (i.e., fusion beats). Another advantage of this combinationis that rate chamber pacing can provide backup pacing when in asynchronized chamber-only pacing mode, such that when the synchronizedchamber pace is prevented, for example to avoid pacing during thechamber vulnerable period following a prior contraction, the ratechamber will not be pseudo-paced and thus will be paced upon expirationof the rate chamber escape interval. Synchronized chamber-only pacingcan be combined also with a triggered synchronized pacing mode, inparticular with the type in which the synchronized chamber is triggeredby a sense in the rate chamber. One advantage of this combination isthat sensed events in the rate chamber will trigger the synchronizedchamber-only pacing, which may benefit some patients by synchronizingthe paced chamber contractions with premature contralateral intrinsiccontractions.

[0030] An example of synchronized chamber-only pacing is leftventricle-only synchronized pacing where the rate and synchronizedchambers are the right and left ventricles, respectively. Leftventricle-only synchronized pacing may be advantageous where theconduction velocities within the ventricles are such that pacing onlythe left ventricle results in a more coordinated contraction by theventricles than with conventional right ventricular pacing orbiventricular pacing. Left ventricle-only synchronized pacing may beimplemented in inhibited demand modes with or without atrial tracking,similar to biventricular pacing. A left ventricular pace is thendelivered upon expiration of the AVI escape interval or expiration ofthe LRL escape interval, with left ventricular pacing inhibited by aright ventricular sense that restarts the LRL escape interval and stopsthe AVI escape interval.

[0031] In the synchronized modes described above, the rate chamber issynchronously paced with a mode based upon detected intrinsic activityin the rate chamber, thus protecting the rate chamber against pacesbeing delivered during the vulnerable period. In order to providesimilar protection to a synchronized chamber or synchronized pacingsite, a synchronized chamber protection period (SCPP) may be provided.(In the case of multi-site synchronized pacing, a similar synchronizedsite protection period may be provided for each synchronized site.) TheSCPP is a programmed interval which is initiated by sense or paceoccurring in the synchronized chamber during which paces to thesynchronized chamber are inhibited. For example, if the right ventricleis the rate chamber and the left ventricle is the synchronized chamber,a left ventricular protection period LVPP is triggered by a leftventricular sense which inhibits a left ventricular pace which wouldotherwise occur before the escape interval expires. The SCPP may beadjusted dynamically as a function of heart rate and may be differentdepending upon whether it was initiated by a sense or a pace. The SCPPprovides a means to inhibit pacing of the synchronized chamber when apace might be delivered during the vulnerable period or when it mightcompromise pumping efficiency by pacing the chamber too close to anintrinsic beat. In the case of a triggered mode where a synchronizedchamber sense triggers a pace to the synchronized chamber, the pacingmode may be programmed to ignore the SCPP during the triggered pace.Alternatively, the mode may be programmed such that the SCPP starts onlyafter a specified delay from the triggering event, which allowstriggered pacing but prevents pacing during the vulnerable period.

[0032] In the case of synchronized chamber-only synchronized pacing, asynchronized chamber pace may be inhibited if a synchronized chambersense occurs within a protection period prior to expiration of the ratechamber escape interval. Since the synchronized chamber pace isinhibited by the protection period, the rate chamber is not pseudo-pacedand, if no intrinsic activity is sensed in the rate chamber, it will bepaced upon expiration of the rate chamber escape intervals. The ratechamber pace in this situation may thus be termed a safety pace. Forexample, in left ventricle-only synchronized pacing, a right ventricularsafety pace is delivered if the left ventricular pace is inhibited bythe left ventricular protection period and no right ventricular sensehas occurred.

[0033] As noted above, synchronized pacing may be applied to multiplesites in the same or different chambers. The synchronized pacing modesdescribed above may be implemented in a multi-site configuration bydesignating one sensing/pacing channel as the rate channel forsensing/pacing a rate site, and designating the other sensing/pacingchannels in either the same or the contralateral chamber as synchronizedchannels for sensing/pacing one or more synchronized sites. Pacing andsensing in the rate channel then follows rate chamber timing rules,while pacing and sensing in the synchronized channels followssynchronized chamber timing rules as described above. The same ordifferent synchronized pacing modes may be used in each synchronizedchannel.

[0034] 4. Ventricular Rate Regularization

[0035] Ventricular rate regularization (VRR) is a ventricular pacingmode in which the LRL of the pacemaker is dynamically adjusted inaccordance with a detected intrinsic ventricular rate. When a pacemakeris operating in a ventricular pacing mode (e.g., VVI or DDD), intrinsicventricular beats occur when the instantaneous intrinsic rate risesabove the LRL of the pacemaker. Thus, paces are interspersed withintrinsic beats, and the overall ventricular rhythm as a result of bothpaces and intrinsic beats is determined by the LRL and the mean valueand variability of the intrinsic ventricular rate. VRR regularizes theoverall ventricular rhythm by adjusting the LRL of the pacemaker inaccordance with changes in the measured intrinsic rate.

[0036]5. Pace Counter Isolation

[0037] Resynchronization therapy is only as effective as to the extentto which paces are delivered and not inhibited by intrinsic activity. Inorder to provide a clinician with diagnostic information enabling properadjustment of pacing parameters to optimally deliver cardiacresynchronization therapy, data needs to be collected by the pacemakerwhich reflects the events responsible for causing paces to be deliveredor inhibited over a period of time. In accordance with the presentinvention, isolated pace counters are provided for separately countingdifferent pacing events in each pacing channel. Depending upon thepacing mode which is being used, separate counters are provided forseparately counting paces due to expiration of different escapeintervals and triggering events. In a ventricular resynchronizationpacing mode, for example, counters are provided for separately countingpaces delivered to rate ventricular rate chamber due to expiration of aventricular escape interval corresponding to the lower rate limitsetting and due to expiration of the atrio-ventricular interval in anatrial tracking mode. Separate counters are also provided to count pacesdelivered to the rate and/or synchronized ventricles that are triggeredby ventricular senses in a ventricular triggered mode. Separate countsof senses from each sensing channel may be also maintained.

[0038]FIGS. 2 through 5 illustrate examples of how the separate pacecounters can be used for diagnostic purposes in order to optimallydeliver ventricular resynchronization therapy. Each figure shows thesense and pace counts as they would be collected by non-separatedcounters, and the same data as reflected by isolated counters inaccordance with the invention. The counts are shown for sensing andpacing events in each ventricular sensing/pacing channel. In all cases,the counts are presented as a percentage of total cardiac cycles.

[0039]FIG. 2 shows exemplary counts collected by a pacemaker operatingin an atrial tracking mode with one ventricular sensing/pacing channel.It is desirable in such a situation that ventricular paces should trackthe atria as much as possible. The non-separated counter column showsthat the ventricle is being 100% paced. However, the isolated countersreveal that the reason for the ventricular pacing is all due toexpiration of the LRL escape interval. This indicates that the LRLsetting is inappropriately programmed in order for the pacing rate to becontrolled by intrinsic atrial activity.

[0040]FIG. 3 shows another example of data collected by a pacemaker withone sensing/pacing channel operating in an atrial tracking mode. Thenon-separated counters indicate that the ventricles are being 100%paced, which is what is desired in ventricular resynchronization pacing.However, the separated pace counters reveal that many of the ventricularpaces are not occurring at the expiration of the atrio-ventricularinterval indicating too high an atrial rate, the effect of VRR, or othercondition interfering with proper AV sequential pacing.

[0041]FIG. 4 shows data collected by a pacemaker with right and leftventricular sensing channels operating in a left ventricular-only pacingmode that includes ventricular rate regularization and biventriculartriggering. FIG. 5 shows the same data as in FIG. 4, but presented toshow the total pace counts due to all events for each ventricularchannel and separate counts for each type of pacing event expressed as apercentage of the total paces in that channel. In this example, what isdesired is left ventricular pacing due to expiration of the LRLventricular escape interval. The non-separated counters show that theleft ventricle is paced frequently, but not at the ideal of 100%. Also,the percentage of right ventricular paces is quite high, and it is clearthat this is due to frequent left ventricular senses occurring (i.e.,right ventricular safety paces are being delivered). The separatedcounters help to diagnose the situation. The right ventricular countersreveal that there has been a right ventricular safety pace on 25% of thecycles which indicated oversensing of the left ventricle. The rightventricular counters also show that 60% of the cycles were triggeredright ventricular paces, while the left ventricular counters show thatonly 45% of the cycles had a triggered left ventricular pace. Thisfurther indicates a left ventricular oversensing situation. Finally, itcan be seen that only 15% of the cycles were of the type desired, a leftventricular pace due to expiration of the LRL escape interval.

[0042] The above-described embodiments dealt with ventricularresynchronization pacing modes in which the right and left ventricleswere designated as the rate and synchronized chambers, respectively.Embodiments may similarly be constructed in which contralateral heartchambers are designated as the rate and synchronized chambers or inwhich a plurality of synchronized channels are utilized to providesynchronized pacing to multiple sites of a single chamber. In each ofthese cases, separate counters may be maintained for each sensing/pacingchannel.

[0043] Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. A method for operating a cardiac rhythmmanagement device, comprising: sensing depolarizations and deliveringpaces to one or more heart chambers through one or more sensing/pacingchannels; wherein a heart chamber is paced in accordance with a pacingmode in which a pace is delivered upon expiration of an escape intervalor as a result of a triggering event; for each sensing/pacing channel,separately counting the number of senses received, the number of pacesdelivered by reason of expiration of an escape interval, and the numberof paces triggered by a triggering event.
 2. The method of claim 1wherein one or both ventricles are paced and sensed, the escape intervalcorresponds to the programmed lower rate limit, and the triggering eventis a ventricular sense.
 3. The method of claim 2 further comprising:sensing atrial depolarizations through an atrial sensing channel andgenerating atrial sense signals in accordance therewith, wherein thepacing mode further includes pacing the ventricle upon expiration of anatrio-ventricular interval triggered by an atrial sense; and, separatelycounting the number of ventricular paces delivered by reason ofexpiration of the atrio-ventricular interval.
 4. The method of claim 2wherein the pacing mode is a biventricular resynchronization pacingmode.
 5. The method of claim 2 wherein the pacing mode is a ventricularsynchronized chamber-only pacing mode.
 6. The method of claim 2 whereinthe pacing mode further includes ventricular rate regularization.
 7. Amethod for operating a cardiac rhythm management device, comprising:sensing depolarizations and delivering paces to one or both ventriclesthrough separate sensing/pacing channels; sensing atrial depolarizationsthrough an atrial sensing channel; wherein a ventricle is paced inaccordance with a pacing mode in which a pace is delivered uponexpiration of a ventricular escape interval corresponding to theprogrammed lower rate limit or upon expiration of an atrio-ventricularinterval triggered by an atrial sense; and, for each ventricularsensing/pacing channel, separately counting the number of ventricularsenses received, the number of paces delivered by reason of expirationof the ventricular escape interval, and the number of paces delivered byreason of expiration of the atrio-ventricular interval.
 8. The method ofclaim 7 wherein the pacing mode is a biventricular resynchronizationpacing mode.
 9. The method of claim 7 wherein the pacing mode is asynchronized chamber-only pacing mode.
 10. The method of claim 7 whereinthe pacing mode further includes ventricular rate regularization.
 11. Acardiac rhythm management system, comprising: one or more ventricularsensing/pacing channels for delivering paces and sensing ventriculardepolarizations; an atrial sensing channel for sensing atrialdepolarizations; a controller for controlling the delivery of paces toeach ventricle in accordance with a programmed pacing mode in which apace is delivered upon expiration of a ventricular escape intervalcorresponding to the programmed lower rate limit or upon expiration ofan atrio-ventricular interval triggered by an atrial sense; and, whereinthe controller is further programmed to, for each ventricular sensingchannel sensing/pacing channel, separately count the number ofventricular senses received through each sensing channel, the number ofpaces delivered by reason of expiration of the ventricular escapeinterval, and the number of paces delivered by reason of expiration ofthe atrio-ventricular interval.
 12. The system of claim 11 wherein thecontroller is further programmed to pace the ventricle upon detection ofa ventricular sense in a ventricular-triggered mode and to count thenumber of ventricular paces triggered by a ventricular sense for eachventricular sensing/pacing channel.
 13. The system of claim 11 furthercomprising an external programmer with an associated display and atelemetry interface for transmitting the counts to the externalprogrammer, wherein the external programmer is configured to displayseparate counts for each sensing/pacing channel as a percentage of totalcardiac cycles.
 14. The system of claim 13 wherein the externalprogrammer is configured to display total pace counts due to all eventsfor each ventricular sensing/pacing channel and separate counts for eachtype of pacing event expressed as a percentage of the total paces inthat channel.
 15. A cardiac rhythm management system, comprising: one ormore ventricular sensing/pacing channels for sensing ventriculardepolarizations and generating ventricular sense signals in accordancetherewith; a controller for controlling the delivery of paces to aventricle in accordance with a programmed pacing mode in which a pace isdelivered upon expiration of a ventricular escape interval correspondingto the programmed lower rate limit or upon detection of a ventricularsense in a ventricular-triggered mode; and, wherein the controller isfurther programmed to, for each ventricular sensing/pacing channel,count the number of ventricular senses received, count the number ofpaces delivered by reason of expiration of the ventricular escapeinterval, and count the number of paces triggered by a ventricularsense.
 16. The system of claim 15 further comprising an externalprogrammer with an associated display and a telemetry interface fortransmitting the sense and pace counts to the external programmer,wherein the external programmer is configured to display separate countsfor each sensing/pacing channel as a percentage of total cardiac cycles.17. The system of claim 16 wherein the external programmer is configuredto display total pace counts due to all events for each ventricularsensing/pacing channel and separate counts for each type of pacing eventexpressed as a percentage of the total paces in that channel.